1. Field of the Invention
The present invention relates to a cathode ray tube used for displaying various kinds of images and a signal-detecting method for detecting a signal used for correcting image-display or the like in the cathode ray tube.
2. Description of the Related Art
A cathode ray tube (CRT) is widely used in an image-display apparatus (such as a television receiver, various monitors, and the like). In the CRT, an electron beam is emitted from an electron-gun provided in the tube toward a phosphor screen and is electromagnetically deflected by a deflection yoke or the like, thereby forming a scan image according to the scan with the electron beam on the tube screen.
A CRT having a single electron-gun is common. In recent years, a CRT having a plurality of electron-guns also is being developed. For example, a CRT for displaying a color image having two electron-guns each for emitting three electron beams of red (R), green (G), and blue (B) has been developed. In the CRT of an in-line electron-gun type, a plurality of split-picture planes are formed by a plurality of electron beams emitted from the plurality of electron-guns and are joined, thereby forming a single picture plane. In such a manner, an image is displayed. For example, the technique related to the CRT of the in-line, electron-gun type is disclosed in Japanese Patent Laid-open No. Sho 50-17167, and the like. Such a CRT of the in-line electron-gun type has an advantage that a larger screen can be achieved while reducing the depth as compared with a CRT using a single electron-gun.
It is conventionally known that the display state of an image on a CRT changes according to use conditions. For example, according to environments under which a CRT is used, an influence of an external magnetic field, such as the earth""s magnetic field, varies, and it causes distortion in an image called xe2x80x9craster distortionxe2x80x9d. In the case of a CRT for displaying a color image, a color shift called xe2x80x9cmisconvergencexe2x80x9d in which a scan position of an electron beam for each color shifts on a tube screen, or the like occurs. Such a change in the display state exerts an adverse influence on the precision of the display state in the joint portion in the in-line electronic-gun-type CRT.
The inventor herein has proposed, in Japanese Patent 3068115 and the like, a technique of outputting an electric detection signal in accordance with a scan position of an electron beam and using the detection signal to correct a display state of an image. In the proposal, an electrode called an index electrode is provided in an overscan area of the electron beam in the tube and an electric signal is output from the index electrode in accordance with incidence of an electron beam (hereinbelow xe2x80x9celectric index methodxe2x80x9d). In the electric index method, an electric signal detected by the index electrode is output to the outside of an envelope via a capacitor formed by using a part of the envelope forming the CRT. By using the electric index method, the path of the scan of an electron beam can be directly detected. By using the electric index method, a displacement of the path of an electron beam in each of various positions can be measured, so that there is the advantage that the display state of an image can be finely corrected.
An electric signal extracted by the above-mentioned electric index method (hereinbelow, also called an xe2x80x9cindex signalxe2x80x9d) includes, as shown in FIG. 1, not only necessary information regarding the path of the electron beam (hereinbelow, also called xe2x80x9cindex informationxe2x80x9d) but also various unnecessary signal components included in an anode voltage. A portion indicated by reference numeral 202 in FIG. 1 denotes a signal portion indicative of the index-information required. FIG. 2 is an enlarged view of the signal portion 202 indicative of the index information. The unnecessary signal components include, for example, a flyback transformer for generating a high voltage and H-cycle (horizontal-cycle) pulses generated by a deflection yoke (reference numeral 201 in FIG. 1).
As shown in FIGS. 1 and 2, although the value of the horizontal-cycle pulse signal as an unnecessary signal component is, for example, 10V or higher, the value of the pulse signal indicative of the necessary index-information is only about 100 mV. When the signal value of the unnecessary signal component is much larger than that of the necessary signal component, the SN (signal-to-noise) ratio deteriorates and deterioration in precision in extracting the necessary index-information is caused. The deterioration in precision in extracting the index-information exerts an influence on the precision of correction in the case where the index-information is used for correcting the display state of an image.
As a method of extracting the index-information with high precision, a method of eliminating unnecessary waveform components by using a circuit such as a filter can be considered. In the case of extracting a signal with high precision by using the filter circuit, however, the problem that a very complicated circuit is necessary arises.
The present invention has been achieved in consideration of the above-described problems, and its object is to provide a cathode ray tube capable of accurately extracting a necessary signal component from an electrical detection signal that is output in accordance with the incidence of an electron beam without using a complicated circuit configuration and a signal-detecting method in the cathode ray tube.
A cathode ray tube according to the invention has: an envelope; an anode voltage portion to which an anode voltage is supplied; an electron-gun for emitting an electron beam for scanning a valid picture-plane area and an overscan area outside of the valid picture-plane area; first signal-output means for outputting an electric detection signal according to the incidence of the electron beam for scanning the overscan area; and second signal-output means connected to the anode voltage portion, for outputting a reference signal according to the anode voltage. The cathode ray tube according to the invention further includes signal-extracting means for eliminating an unnecessary signal component included in the detection signal on the basis of the reference signal and extracting and outputting a necessary signal component.
A signal-detecting method in a cathode ray tube according to the invention is applied to a cathode ray tube having an envelope, an anode voltage portion to which an anode voltage is supplied, and an electron-gun for emitting an electron beam for scanning a valid picture-plane area and an overscan area outside of the valid picture-plane area. The signal-detecting method according to the invention, in such a cathode ray tube, includes the steps of outputting an electric detection signal in accordance with the incidence of the electron beam for scanning the overscan area; outputting a reference signal according to the anode voltage supplied to the anode voltage portion; eliminating an unnecessary signal component included in the detection signal on the basis of the reference signal; and extracting and outputting a necessary signal component.
In the cathode ray tube and the signal-detecting method in the cathode ray tube according to the invention, the electric detection signal is output in accordance with the incidence of the electron beam scanning the overscan area, and the reference signal according to the anode voltage supplied to the anode voltage portion is output. An unnecessary signal component included in the detection signal is eliminated on the basis of the reference signal, and the necessary signal component is extracted and output.
Other and further objects, features and advantages of the invention will appear more fully from the following description.